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Recent improvements in light-emitting diode (LED) technology has allowed for the use of LEDs for solar simulators with excellent characteristics. In this paper, we present a solar simulator prototype fully based on LEDs. Our prototype has been designed specifically for light soaking and current-voltage (I(V)) measurements of amorphous silicon solar cells. With 11 different LED types, the spectrum from 400 to 750 nm can be adapted to any reference spectrum-such as AM1.5g-with a spectral match corresponding to class A+ or better. The densely packed LEDs provide power densities equivalent to 4 suns for AM1.5g or 5 suns with all LEDs at full power with no concentrator optics. The concept of modular LED blocks and electronics guarantees good uniformity and easy up-scalability. Instead of cost-intensive LED drivers, low-cost power supplies were used with current control, including a feedback loop on in-house developed electronics. This prototype satisfies the highest classifications (better than AAA from 400 to 750 nm) with an illuminated area of 18 cm × 18 cm. For a broader spectrum, the spectral range could be extended by using other types of LEDs or by adding halogen lamps. The space required for this can be saved by using LEDs with higher power or by reducing the maximum light intensity.